Enantioselective total synthesis of pladienolide B: A potent spliceosome inhibitor

Article abstract of DOI:10.1021/ol301886g

An enantioselective and convergent total synthesis of pladienolide B (1) is described. Pladienolide B binds to the SF3b complex of a spliceosome and inhibits mRNA splicing activity. The synthesis features an epoxide opening reaction, an asymmetric reduction of a β-keto ester, and a cross metathesis strategy for the side chain synthesis.

Full text of DOI:10.1021/ol301886g

ORGANIC
LETTERS
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Vol. XX, No. XX
000–000
Enantioselective Total Synthesis
of Pladienolide B: A Potent
Spliceosome Inhibitor
Arun K. Ghosh* and David D. Anderson
Department of Chemistry and Department of Medicinal Chemistry, Purdue University,
560 Oval Drive, West Lafayette, Indiana 47907, United States
akghosh@purdue.edu
Received July 9, 2012
ABSTRACT
An enantioselective and convergent total synthesis of pladienolide B (1) is described. Pladienolide B binds to the SF3b complex of a spliceosome
and inhibits mRNA splicing activity. The synthesis features an epoxide opening reaction, an asymmetric reduction of a β-keto ester, and a cross
metathesis strategy for the side chain synthesis.
In 2004, Sakai et al. reported the isolation and structural
characterization of seven novel 12-membered macrocyclic
compounds named pladienolide AꢀG from a culture of an
engineered strain of Streptomyces platensis, Mer-11107.1
Initial screening studies showed these compounds were
capable of inhibiting the proliferation of human cancer
cells with low nanomolar IC50 values.2 Interestingly, pla-
dienolides maintained this activity against multiple drug
resistant cancer cells. Furthermore, they exhibited a novel
mechanism of action by binding to the SF3b subunit of
the spliceosome, inhibiting the splicing of pre-mRNA to
translatablemRNA. The unspliced pre-mRNAisexported
from the nucleus to the cytoplasm resulting in inhibition of
cellgrowth.3 To date, the pladienolidesand the structurally
distinct FR901464 are the only known molecular scaffolds
capable of modulating splicing and generating an anti-
tumor response. Consequently, these compounds have
shown tremendous potential to become a new class of
anticancer agents. In 2008, clinical trials were initiated
using E7107, a pladienolide D analog, for the treatment
of various cancers.4 Not surprisingly, the chemistry and
biology of the pladienolides attracted immense attention.
To date, Kotake et al. reported the first total synthesis of
the most active compounds, pladienolide B and D.5
Skaanderup et al. published a synthetic pathway to the
macrocyclic core of the enantiomer of pladienolide B,
while Burkart et al. have reported their efforts on con-
structing and modifying the side chain.6 Recently, both
Maier7a and Webb7b have reported progress in the devel-
opment of pladienolide based analogs albeit with reduced
biological activity. Our interest in pladienolide B arose
(1) (a) Sakai, T.; Sameshima, T.; Matsufuji, M.; Kawamura, N.;
Dobashi, K.; Mizui, Y. J. Antibiot. 2004, 57, 173–179. (b) Sakai, T.; Asai,
N.; Okuda, A.; Kawamura, N.; Mizui, Y. J. Antibiot. 2004, 57, 180–187.
(c) Asai, N.; Kotake, Y.; Niijima, J.; Fukuda, Y.; Uehara, T.; Sakai, T.
J. Antibiot. 2007, 60, 364–369.
(2) Mizui, Y.; Sakai, T.; Iwata, M.; Uenaka, T.; Okamoto, K.;
Shimizu, H.; Yamori, T.; Yoshimatsu, K.; Asada, M. J. Antibiot.
2004, 57, 188–196.
(3) (a) Kotake, Y.; Sagane, K.; Owa, T.; Mimori-Kiyosue, Y.; Shimizu,
H.; Uesugi, M.; Ishihama, Y.; Iwata, M.; Mizui, Y. Nat. Chem. Biol. 2007,
3, 570–5. (b) Yokoi, A.; Kotake, Y.; Takahashi, K.; Kadowaki, T.;
Matsumoto, Y.; Minoshima, Y.; Sugi, N. H.; Sagane, K.; Hamaguchi,
M.; Iwata, M.; Mizui, Y. FEBS J. 2011, 278, 4870–80.
(4) Butler, M. A. Natural Product-Derived Compounds in Late Stage
Clinical Development at the End of 2008; Buss, A., Butler, M., Eds.; The
Royal Society of Chemistry: Cambridge, U.K., 2009; pp 321ꢀ354.
(5) Kanada, R. M.; Itoh, D.; Nagai, M.; Niijima, J.; Asai, N.; Mizui,
Y.; Abe, S.; Kotake, Y. Angew. Chem., Int. Ed. 2007, 46, 4350–5.
(6) (a) Skaanderup, P. R.; Jensen, T. Org. Lett. 2008, 10, 2821–2824.
(b) Mandel, A. L.; Jones, B. D.; La Clair, J. J.; Burkart, M. D. Bioorg.
Med. Chem. Lett. 2007, 17, 5159–5164.
(7) (a) Muller, S.; Mayer, T.; Sasse, F.; Maier, M. E. Org. Lett. 2011,
13, 3940–3943. (b) Gundluru, M. K.; Pourpak, A.; Cui, X.; Morris,
S. W.; Webb, T. R. MedChemComm 2011, 2, 904–908.
r
10.1021/ol301886g
XXXX American Chemical Society

from its novel mechanism of action and its unique struc-
tural features. We sought to develop a convergent route to
pladienolide B that could facilitate subsequent structureꢀ
activity relationship studies and synthesis of structural
variants. Herein, we report a concise, enantioselective
synthesis of pladienolide B.
As shown in Figure 1, we planned to utilize a late stage
JuliaꢀKocienski olefination8 similar to Kotake et al.5 to
append the epoxide containing side chain 2 to the macro-
cyclic core 3, providing the transꢀtrans diene system. The
hydroxy groups in both intermediates 2 and 3 would be
conveniently protected with silyl ether groups thereby
allowing a global deprotection at the final stage of the
synthesis. Side chain 2 could be efficiently synthesized by
cross metathesis of alcohol 4 and mesylate 5. Shi epoxida-
tion of the resulting olefin would install the desired epoxide
moiety.9 Both chiral alcohol 4 and mesylate 5 are readily
available in optically active form. We planned to construct
the macrocyclic core3 by ring closing metathesis (RCM) of
triene 6 in hopes that the free allylic alcohol would produce
an activating effect resulting in higher yields and shorter
reaction times.10 Of particular relevance, a related RCM
macrocyclization strategy by Kotake et al.5 with a pro-
tected diol provided a low yield and significant amount of
isomerized olefin. Triene 6 in turn could be synthesized by
Yamaguchi esterificationofacid8 and homoallylic alcohol
7.11 The synthesis of alcohol 7 could be achieved by
Brown’s crotylation of the corresponding aldehyde.12 Acid
8 would be synthesized by a ring-opening reaction of
epoxide 9 with the dianion of tert-butyl acetoacetate 10
followed by asymmetric reduction of the resulting β-keto
ester. Epoxide 9 could be synthesized using a desymme-
trization strategy via Sharpless epoxidation.13
The synthesis of the C1ꢀC8 fragment is outlined in
Scheme 1. Asymmetric desymmetrization of commercially
available divinyl carbinol 11 using Sharpless epoxidation
followed by PMB protection of the corresponding alcohol
provided epoxide 9 in 43% yield as a single isomer
according to literature methods.14 Ring-opening of epox-
ide 9 with the dianion of tert-butyl acetoacetate 10 fol-
lowed by silyl protection of the resulting alcohol with
TESCl afforded acyclic β-keto ester 12.15 Asymmetric
reduction of 12 with NaBH4-L-tartaric acid complex as
prepared by Yotagai and Ohnuki16 successfully reduced
Figure 1. Retrosynthetic Analysis of Pladienolide B.
the ketone with excellent stereoselectivity (>95% dr). The
depicted stereochemistry is based on the observed stereo-
chemical outcome of β-keto esters as reported.16 Silyl
protection of the newly formed alcohol provided ester 13
as a single isomer in high yields (61% from 9). Oxidation of
13 with 3.5 equiv of IBX selectively removed the TES ether
and oxidized the alcohol to a ketone without migration of
the terminal olefin.17 The resulting ketone was treated with
MeMgBr in THF at ꢀ78 °C providing the desired tertiary
alcohol 14 as a single isomer (>95% by NMR) in 85%
yield (quant. BRSM). The stereochemistry was assigned
based on Cram’s chelation model.18 Treatment of alcohol
14 with an excess of TESOTf in the presence of Et3N
in CH2Cl2 protected the tertiary alcohol as a TES ether
with concomitant formation of the silyl ester which was
hydrolyzed upon aqueous workup providing acid 8.
(8) Blakemore, P. R.; Cole, W. J.; Kocienski, P.; Morley, A. Synlett
1998, 1, 26–28.
(9) Wang, Z.-X.; Tu, Y.; Frohn, M.; Zhang, J.-R.; Shi, Y. J. Am.
Chem. Soc. 1997, 119, 11224–11235.
(10) Hoye, T.; Zhao, H. Org. Lett. 1999, 1, 1123–1125.
(11) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M.
Bull. Chem. Soc. Jpn. 1979, 52, 1989–1993.
(12) (a) Brown, H.; Bhat, K. J. Am. Chem. Soc. 1986, 108, 5919–5923.
(b) Brown, H.; Bhat, K. J. Org. Chem. 1986, 108, 293–294.
(13) Gao, Y.; Klunder, J. M.; Hanson, R. M.; Masamune, H.; Ko,
S. Y.; Sharpless, B. J. Am. Chem. Soc. 1987, 109, 5765–5780.
(14) Schreiber, S.; Smith, D. J. Org. Chem. 1989, 54, 9–10.
(15) (a) Weiler, L. J. Am. Chem. Soc. 1970, 92, 6702–6704. (b) Lygo,
B.; O’Connor, N.; Wilson, P. R. Tetrahedron 1988, 44, 6881–6888. (c)
Kieczykowski, G.; Roberts, M.; Schlessinger, R. J. Org. Chem. 1978, 43,
788–789.
(17) (a) Frigerio, M.; Santagostino, M.; Sputore, S.; Palmisano, G.
J. Org. Chem. 1995, 60, 7272–7276. (b) Wu, Y.; Huang, J.-H.; Shen, X.;
Hu, Q.; Tang, C.-J.; Li, L. Org. Lett. 2002, 4, 2141–2144.
(16) Yotagai, M.; Ohnuki, T. J. Chem. Soc., Perkin Trans. 1 1990, 6,
1826–1828.
(18) (a) Cram, D. J.; Kopecky, K. J. Am. Chem. Soc. 1959, 81, 2748–
2755. (b) Reetz, M. Angew. Chem., Int. Ed. 2003, 23, 556–569.
B
Org. Lett., Vol. XX, No. XX, XXXX

Our asymmetric synthesis of the C1ꢀC8 segment is quite
efficient and provides convenient access to other possible
stereoisomers. Previous syntheses of this segment5,6a,7a
relied on terpene (nerol and R-linalool) modification
strategies.
Table 1. Asymmetric Reduction of β-Keto Ester
Scheme 1. Synthesis of C1ꢀC8 Intermediate 8
entry
R1
R2
additive
none
S:R
1
2
3
4
5
6
7
8
H
t-butyl
t-butyl
t-butyl
t-butyl
methyl
methyl
methyl
methyl
37:63
15:85
45:55
5:95
H
L-tartaric acid
none
TES
TES
TES
TES
TBS
TPS
L-tartaric acid
L-tartaric acid
D-tartaric acid
L-tartaric acid
L-tartaric acid
8:92
94:6
5:95
5:95
Scheme 2. Synthesis of C9ꢀC13 Intermediate 7
We briefly examined the scope of the NaBH4/tartaric
acid reduction of β-keto ester 12. As shown in Table 1,
reduction of the β-keto ester in the absence of tartaric acid
resulted in only a small degree of selectivity (entry 3). With the
TES group removed (entry 1) the selectivity was increased
suggesting the free hydroxy group had a beneficial directing
effect. When the NaBH4/tartaric acid system was employed,
a further improvement in selectivity was observed (entry 2).
However, the best results were obtained with NaBH4/tartaric
acid with the hydroxy group protected as a bulky silyl ether
suggesting that steric effects were the dominating factor
(entries 5, 7, and 8) for good diastereoselectivity. The size
of the ester group also played an important role with the
bulky tert-butyl providing the best selectivity (compare en-
tries 4 and 5). Interestingly, using D-tartaric acid (entry 6)
gave a similar degree of selectivity for the other diastereomer
suggesting the chirality was transferred from the tartaric acid
and not induced from the chirality inherent to the substrate.
Synthesis of optically active homoallylic alcohol 7 is
shown in Scheme 2. Prenyl alcohol 15 was protected as a
trityl ether with tritylchloride and Et3N in the presenceof a
catalytic amount of DMAP. Riley oxidation of the result-
ing trityl ether provided allylic alcohol 16 in 52% yield
along with some aldehyde 17.19 The resulting mixture was
exposedtoMnO2 oxidation in CH2Cl2 toprovidealdehyde
17 in good overall yield. Brown’s asymmetric crotylation12
of aldehyde 17 furnished homoallylic alcohol 7 in >82% ee
and 64% yield over two steps.
With acid 8 and alcohol 7 in hand we proceeded to form
the macrocyclic ring structure. As shown in Scheme 3,
esterification of acid 8 with alcohol 7 according to
Yamaguchi’s protocol11 followed by DDQ oxidative removal
of the PMB ether provided allylic alcohol 18. RCM with
Grubbs’ second generation catalyst (>95% E by HPLC)
followed by treatment with Ac2O in pyridine furnished
lactone 19.20 Despite the presence of a free allylic alcohol,
the RCM still required elevated temperatures although
higher yields were observed compared to Kotake et al.5,10
Interestingly, when the allylic alcohol was protected as a
PMB ether, RCM was very sluggish. Treatment of 19 with
DDQ for 24 h selectively removed the trityl ether, while
oxidation of the resulting allylic alcohol with IBX afforded
aldehyde 3 in 75% yield over two steps.17 The use of BCl3
at low temperatures was also effective in selectively remov-
ing the trityl ether; however, DDQ provided more robust
conditions.21
(20) (a) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett.
1999, 1, 953–956. (b) Chatterjee, A. K.; Choi, T.-L.; Sanders, D.;
Grubbs, R. J. Am. Chem. Soc. 2003, 125, 11360–11370.
(21) Jones, G.; Hynd, G.; Wright, J.; Sharma, A. J. Org. Chem. 2000,
65, 263–265.
(19) Umbriet, M.; Sharpless, K. J. Am. Chem. Soc. 1977, 99, 5526–
5528.
Org. Lett., Vol. XX, No. XX, XXXX
C

Scheme 3. Synthesis of Macrocyclic Core 3
Scheme 4. Synthesis of Sulfone 2
With the synthesis of the macrocycle core complete, we
then focused on development of a concise route to the epoxide-
containing side chain C12ꢀC23. As shown in Scheme 4, cross
metathesis between homoallylic alcohol 4 and mesylate 5
proceeded in moderate yield with a high degree of selectivity
for the trans olefin (5:1 E/Z).20 Subsequent protection of the
homoallylic alcohol as a TES ether enabled the separation
of olefin 20 from the other undesired cross metathesis
products. Displacement of the mesylate with the anion
of 1-phenyl-1H-tetrazole-5-thiol followed by ammonium
molybdate-catalyzed hydrogen peroxide oxidation gave sul-
fone 21 in 71% yield over two steps. Shi epoxidation9 of sulfone
21 followed by TES protection of the secondary alcohol
afforded the side chain intermediate 2 as a single isomer.
The final assembly of pladienolide B is shown in Scheme 5.
JuliaꢀKocienski olefination8 between sulfone 2and aldehyde
3 provided diene 22 which was subsequently deprotected by
the addition of TBAF yielding pladienolide B. The 1H and 13C
NMR of our synthetic pladienolide B {[R]D23 = þ7.3 (c 0.26,
MeOH)} is identical to the reported spectra for the natural
Scheme 5. Synthesis of Pladienolide B
pladienolide B {Lit.5 [R]D = þ7.9 (c 1.1, MeOH)}, thus
27
confirming the absolute configuration of our synthetic material.
In summary, we have accomplished an enantioselective
synthesis of pladienolide B. The synthetic route is con-
vergent and readily scaleable. Overall, our synthesis in-
volved31total steps from commercially available material.
The longest linear path was 16 steps with an overall yield of
1.4%. This represents a significant improvement over
Kotake’s route which required 59 total steps (22 longest
linear) from commercially available material. The synthe-
sis featured an effective asymmetric reduction of a β-keto
ester, a cross metathesis reaction, and JuliaꢀKocienski
olefinations. Other key reactions included a Sharpless
asymmetric epoxidation and Brown asymmetric crotyl-
boration reactions. The synthesis will provide convenient
access to a variety of derivatives. Further investigations of
structural and biological studies are in progress.
Acknowledgment. Financial support by the National In-
stitutes of Health is gratefully acknowledged. We also thank the
ACS Division of Medicinal Chemistry and the American Foun-
dation for Pharmaceutical Education for predoctoral fellow-
ships to David Anderson. We thank Mr. Khriesto Shurrush
(Purdue University) for preliminary experimental assistance.
Supporting Information Available. Experimental pro-
cedures and 1H and 13C NMR spectra for all new
compounds have been provided. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX